The use of the previously developed adenovirus host-cell plaque assay (in which adenovirus 2 or 5 is treated with physical or chemical agents, and is then assayed for biological activity on monolayers of human cells) was extended: 1) Eight of thirty-one human tumor cell strains were found to be deficient in their ability to repair adenovirus 5 treated with N-methyl-N'-nitro-N-nitrosoguanidine. All of 21 human skin fibroblast strains were proficient in such repair. 2) Hydroxyurea was found to block repair of ultraviolet light damaged human adenovirus 5.